1. Field of the Invention
The invention relates a light detecting element and a control method of the light detecting element.
2. Description of the Related Art
A conventional light detecting element is known by different element names such as a photodiode, phototransistor, CCD image sensor, etc. Such an element is widely utilized in different applications such as a photosensor which detects presence of a physical object based on variation of strength of the received light, optical (light) communication which uses light as transmission medium, distance sensor (rangefinder) which optically measures distance based on triangulation method, phase difference between the issued light and the received light, etc, image pick-up device such as a video camera or digital camera, or the like.
In these applications, there is a case that a light source is used together with the light detecting element such as the photosensor which monitors an intruder, the optical communication such as (infrared) light remote controller, the distance sensor which is used in an auto focus camera or robot eye, the image pick-up device which is used together with the light source in order to obtain a distance image, or the like. In this case, since the element receives not only the light from the light source but also ambient light such as natural light, the strength of the received light increases as compared with a case that only the light from the light source is received.
In this kind of the light detecting element, carriers (electrons or electron holes) are generated within the element by the quantity corresponding to quantity of the received light. However, the generated quantity of the carriers gradually approaches a saturation level as the quantity of the received light increases because there is a limitation in the quantity of the carriers generated according to the quantity of the received light. Therefore, when the ambient light is received together with a signal light which includes object information in an application such as the above, there is a problem in that a large output cannot be obtained from the signal light since dynamic range of the element is reduced by the quantity of the ambient light.
Also, when the signal light is intermingled with the ambient light, there is a possibility that the signal light cannot be distinguished from the intermingled light if the ambient light fluctuates. It is possible to distinguish the signal light from the intermingled light by using an optical filter which passes a specific wavelength components included in the signal light. However, when the ambient light has spectrum components spreading over a wide range such as sun light, influence of the ambient light cannot be sufficiently removed even through the optical filter.
In another prior art for separating the signal light and the ambient light, output of the element is separated into components of the signal light and components of the ambient light. For example, a prior art described in Japanese Laid-open Patent Publication No. 2001-337166 reduces or cancels the components of the ambient light by finding differential components between the light received in a light period and the light received in a extinction period while intermittently operating the light source. The light period is a period of time that the light source is operated, while the extinction period is a period of time that the light source is extinguished (not operated). Namely, in such a short period of time in which the ambient light does not fluctuate, the differential components are calculated by subtracting output corresponding to the received light with only the components of the ambient light in the extinction period from output corresponding to the received light with the components of the ambient light and the signal light in the light period. As a result, it becomes possible to suppress the components of the ambient light to significantly increase rate of the components of the signal light.
However, in the prior art described Japanese Laid-open Patent Publication No. 2001-337166, there is a problem is that the components of the signal light cannot be detected when the element is saturated. This is because the prior art calculates the differential components at an external circuit for the light detecting element in order to separate the output of the element into the components of the signal light and the components of the ambient light. That is, the dynamic range for the signal light of the element becomes narrow under the presence of the ambient light and therefore the prior art cannot still solve the problem that a large output cannot be obtained from the light detecting element according to the signal light.
Expanding upon the above, it is necessary to increase the number of the carriers generated by light irradiation in order to reduce the influence of shot noise. This is because the limitation of detection precision of the element is generally decided by the shot noise attended with photoelectric conversion. When being under the saturation level, the number of the carriers becomes larger as the strength of the received light becomes stronger and also the light receiving time becomes longer (i.e., the quantity of the received light becomes larger). Therefore, it is possible to reduce the influence of the shot noise by increasing the irradiation strength of the light source or lengthening the light receiving time of the light detecting element. However, the SN ratio cannot be sufficiently increased even though the irradiation quantity of the source is increased or the light receiving time of the element is lengthened. This is because the dynamic range of the element is reduced under the presence of the ambient light.
The prior art also requires reading out at least two times from an image pick-up device such as a CCD sensor or an MOS type sensor (i.e., the light detecting element). This is because the output corresponding to the received light must be picked out twice from the image pick-up device in order to take the differential components. Namely, because read out time of 2 frames is taken until a detection result is obtained, its response speed is reduced by the time.